Question:

Whether a variable is declared with the var keyword or not determines the scope of that variable. Describe how this mechanism works and give at least one example of how this works.

You should always declare a variable with the var word to not imply a global variable. You might be wondering why would this happen when I didn’t tell JavaScript I wanted it to be global. Let’s see :

var y = 20;

function multiply() {
 var x = 10;
 y ;
 
 console.log(x * y);
 
}
multiply(); // output: 200

Above we have y thats declared in the multiply function but since we didn’t declare with var y goes up to global in gets the global y value. If we change y to have a value then it will override the global variable which could lead to problems if we wanted y to stay 20 globally. If we didn’t have a global variable set up then y would still become a global variable.

var y = 20;

function multiply() {
 var x = 10;
 y = 10 ;
 
 console.log(x * y);
 
}
multiply(); // output: 100 oh no the global variable was overwritten 

Now you know why you should declare your variables with var so you don’t end up changing your global object.